Abartach

Fionn Cycle

In the Fenian tale The Pursuit of the Gilla Decair and His Horse, the wizard Abartach mac Alchad was able to make a number of the Fianna disappear, by leading them on a journey to Tìr Tairnigir, the Land of Promise in the Otherworld, while riding a grey horse. When Goll mac Morna and Oscar, grandson of Fionn, rescued their comrads, they instituted retribution against Abartach, which included the theft of his wife. However, Abartach was able to use his magic and disappear without paying.

What is interesting is the recurrence of the theme of a grey (sometimes white) horse as the magical transporter between this world and the other world; it is another magical horse which brings Oisín back from the Otheworld in another Fenian romance, Acallam na Senórach.

SOURCES
"The Pursuit of the Gilla Decair and His Horse." Silva Gaedelica. ed. and trans. O'Grady, Standish. WWW: In Parenthesis, http://www.yorku.ca/inpar/gilla_ogrady.pdf.